The invention pertains to optical devices for lightwave communications. Particularly, the invention pertains to electro-optic modulators on absorptive substrates and, more particularly, pertains to such modulators having multifunctionality, particularly lightwave modulation and photodetection.
Any electro-optic modulator incorporating optical waveguides is well known in the art. In lightwave communications, light is modulated with electric signals and is transmitted to a receiver along some desired path, usually on an optical fiber. This constitutes a lightwave link whose function is to communicate the electric signals at the transmitter to electric signals at the receiver. The receivers and transmitters are two separate components of the lightwave link in the art. Lightwave transmitters are usually classified as directly modulating transmitters or as indirectly modulating transmitters. Directly modulating transmitters are typically semiconductor laser transducers which convert the impressed electric signal directly into lightwave signals. Indirectly modulating transmitters consist of three devices: a laser which converts electric energy into lightwave energy, a path to conduct the lightwave energy from the laser to the modulator, and a modulator which converts electric signals into lightwave signals. In this case, the laser is a transducer optimized to generate light but not modulate light, and the modulator is a transducer optimized to modulate light but not generate light. This permits independent control and optimization of the light generation function and of the light modulation function. It is well known in the art that compromises in link performance, reliability, and lifetime may occur if these two functions are combined in one transducer. For example, to obtain high modulation bandwidth, it is necessary to drive the laser with high current which reduces the reliability and lifetime. However, it is also well known in the art that compromises in transmitter cost, size and weight may occur if these two functions are separated. The cost of this indirect modulation configuration is high because there are more packaged parts and the connection between these parts requires optical packaging. In particular, a fiber coupling is required between the laser output and the input of its pigtail fiber, between the output of the laser's pigtail fiber and the input to the modulator's input pigtail fiber, between the output of the modulator and the input of its output pigtail fiber, and finally, between the output of the modulator's output pigtail fiber and the input to the fiber link. While the fiber-to-fiber coupling is relatively inexpensive, the couplings between the laser and its pigtail and the modulator and its pigtails are very expensive. Consequently, fabrication of the modulator is very expensive.
Therefore, to obtain desirable link performance, reliability, and lifetime for some applications, it is necessary to reduce the transmitter cost, size and weight by increasing the level of integration by combining several distinct functions into a single element. In particular, reducing the number of fiber couplings and the number of components required to receive and transmit light signals reduces the cost of fabrication. Chip layout space can also be reduced. Combining the receiver and transmitter into one element is a desirable approach. Combining the receiver and laser into one element has been difficult in the art, because the receiver and the laser have conflicting design constraints and requirements.
Also a desirable feature of the modulator is feedback stabilization. In the art, a tap off means is used to tap a small fraction of the light signal traveling through the waveguide or fiber. This tapped-off signal is sent to a circuit for feedback stabilization. Feedback stabilization usually requires one optical fiber and two fiber couplers for the transmittal of the tapped-off signal to the feedback circuit. Such tap-off means requires costly fiber coupling and much chip layout space. The tap-off technique also requires power from the light signal which reduces the available lightwave signal at the receiver, thus degrading the link signal-to-noise ratio.